This disclosure relates generally to the field of high voltage power supplies that may be used in devices such as radiation generators. More specifically, the disclosure relates to voltage regulation methods and devices for maintaining high voltage within predetermined tolerances notwithstanding changes in load on the high voltage power supply and environmental conditions to which the instrument having the high voltage power supply is exposed.
Electronic radiation generators, for example, (including pulsed neutron generators and x-ray generators) require high voltages for the production of the desired nuclear radiation. These voltages, which can exceed 100 kV DC, and may be as much as 500 kV, are produced by power supplies typically of the Cockcroft-Walton voltage multiplier design. The simplest Cockcroft-Walton voltage multiplier design includes an array or ladder-like structure of stages where, under an AC drive signal (e.g., Us in the schematic below) the voltage at the output of the n-th stage is n times the peak-to-peak value of Us.). Each stage includes discrete components, specifically (2) capacitors and (2) diodes in the arrangement shown in the electrical schematic diagram in FIG. 1. Also included is a voltage divider (or “bleed”) resistor chain (not shown) that may be used to measure the power supply output voltage.
It is known in the art to use a regulation loop to control the high voltage which may or may not include a proportional-integral-derivative (PID) regulation loop. Most types of control loops use an error term related to the difference between a high voltage monitor, which may be measured using a voltage divider with a bleed resistor, and a high voltage set point. Such control loop may be implemented in analog or digital form. The control loop adjusts the drive voltage, defined by Us in FIG. 1, to cause the power supply to generate the desired high voltage output. One limitation of known control loops arises when the load current (e.g., beam current in an x-ray or neutron generator) varies rapidly (i.e. faster than the response time of the control loop) temporally. The changes in load cause the high voltage output to vary, which may stress the high voltage system including the components that make up the ladder and the insulation system surrounding the high voltage power supply. As a hypothetical example, if 5 kV control signal (Us) were used to generate 100 kV power supply output, under heavy load and with an insulation system designed to withstand 110 kV, if the load abruptly dropped, the power supply high voltage output could quickly increase to 120 kV if the effective output impedance of the high voltage power supply is large. This could cause the insulation system or power supply components to be damaged, perhaps permanently.
Conversely, when there is a sudden increase in load current, the high voltage could drop, and in the case of an electrically operated radiation generator, the radiation output and possibly the measurement made by an instrument using such generator could be affected.
To mitigate the foregoing limitation, it is known in the art to use a hardware-embodied circuit to monitor the load current, which tends to lead the change in output voltage, and add a proportional voltage to the drive voltage (Us) or otherwise modify a drive signal such as drive frequency or drive duty factor. In the event that the load current is reduced, the proportional voltage or drive signal would be reduced, thus mitigating the risk of overshoot and subsequent damage to the high voltage system. Corresponding, opposite changes to the drive signal may be applied in the event the load current increases. An example hardware embodied circuit to control the drive signal can be observed in FIG. 2.